Because everyone who does do this will provide a better user interface than you do. You can choose to make your life easier or your users experience nicer.

Personally, my strategy is simply to not write HTML or CSS. I write to a DSL that is implemented based on what browser is being served, so there are no sniffer scripts in the pages themselves. True, this does mean the same, otherwise static pages, may need to get generated over and over, but this is where caching comes in.